This invention relates to an improved particle analyzer and more particular to such a particle analyzer adapted for measuring the resistance and reactance of a particle, such as a blood cell, in order to determine the electrical opacity of the particle.
As is well known in the art, the electrical opacity of a blood cell has been defined as the ratio of the a.c. impedance to the d.c. resistance of the cell. Apparatus to provide data which enables the electrical opacity of the cell to be measured was first proposed in U.S. Pat. No. 3,502,974 in the name of Wallace H. Coulter and Walter R. Hogg and entitled "Signal Modulated Apparatus For Generating And Detecting Resistive And Reactive Changes In A Modulated Current Path For Particle Classification And Analysis". Such opacity parameter can be used in a number of different fashions to obtain certain results when analyzing blood cells. Examples of use of opacity are described in U.S. Pat. No. 4,298,836 in the name of Michael R. Groves et al and entitled "Particle Shape Determination", U.S. Pat. No. 4,525,666 in the name of Michael R. Groves and entitled "Cell Breakdown" and U.S. Pat. No. 4,535,284 in the name of Michael R. Groves et al entitled "High And Low Frequency Analysis Of Osmotic Stress Of Cells". Each of the above noted U.S. patents have been assigned to the assignee of the present invention. In addition, uses for opacity have been described PCT Published Application WO85/05684, also assigned to the assignee hereof, and in the articles entitled "Flow System Measurement Of Cell Impedance Properties", R. A. Hoffman and W. B. Britt, The Journal of Histrochemistry and Cytochemistry, Volume 27, Number 1, pages 234-240 (1979) and "Two Dimensional Impedance Studies of BSA Buoyant Density Separated Human Erythrocytes", R.C. Leif, et al. Cytometry, Volume 6, Pages 13-21 (1985).
A unique apparatus and principle of blood cell counting and sizing was invented by Wallace H. Coulter and is described in U.S. Pat. No. 2,656,508. According to the Coulter principle, a fluid electrolyte containing particles, such as blood cells, passes from one chamber to another chamber through a small orifice or aperture. An electrode is placed in each of the chambers and a direct current, or low frequency current is applied to the electrodes and through the orifice, thereby creating an electric field in the orifice. As a particle, or blood cell, traverses the orifice, the resistance within the orifice is changed. This resistance can be sensed by sensing the voltage across the electrodes, whereby the presence of a particle in the orifice causes a pulse in the voltage being sensed by the electrodes.
The Coulter principle, first described in the aforementioned U.S. Pat. No. 2,656,508, has been expanded by additionally providing a high frequency current through the orifice at the same time that the low frequency, or direct current, signal is passed through the orifice. By appropriate filtering techniques, both the low frequency resistance and high frequency reactances of the cell traversing the orifice can be detected. Such detection has been described in more detail in the aforementioned articles and the United States Patents as the structure used to obtain the data for determining the opacity. In addition, cells can be detected using the high frequency current alone and in so doing, additional information can be obtained based on the fact that the amplitude of the pulse varies in response to the frequency of the current through the orifice.
One of the problems with the prior art structure has been interference created between the two separate current sources used to create the a.c. and d.c. fields within the orifice. One way the prior art teaches coupling the low frequency, or d.c. current source, and the high frequency current source is in parallel to one another and in parallel with the electrodes sensing the particle in the orifice. However, this type of coupling can lead to interference between the two sources. This is particularly true where multiple high frequency oscillators are utilized, such as described in U.S. Pat. No. 3,502,974. Where two high frequency tuned circuits are coupled across the orifice, any slight change in conditions can cause either, or both, of the two frequencies to become detuned. For example, a drift in the temperature or pressure of the fluid, or a bubble within the orifice, can result in problems with the above circuitry. This problem is illustrated in the type of machine described in the article entitled "The Toa Micro Cell Counter" by P. W. Helleman and C. J. Benjamin, Scand. J. Haemat. (1969) 6, pages 69-76, where the oscillator tuned circuit and the detector tuned circuit are separately coupled in parallel in the circuit. Because of this instability, the devices of the prior art have not been able to achieve commercial success, despite having been known for at least fifteen years. In order to render the principle of detecting data sufficient to provide the opacity or high frequency response of a blood cell particle practical, improvements are required in both the oscillator circuitry and means for connecting the oscillator circuitry in circuit with the remainder of the traditional Coulter type transducer.